Lecture: 10 New Trends in Optical Networks Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department

Outline Challenges Multiplexing Techniques Routes to Longer Reach Distributed amplification Hollow core fibers Routes to Higher Transmission Capacity Space division multiplexing (SDM)

The Challenge Traffic grows exponentially at approximately 40% per year Optical system capacity growth has been approximately 20% per year In less than 10 years, current approaches to keep up will not be sufficient Main physical barriers: Channel capacity (Shannon) + available optical bandwidth Transmission fiber nonlinearities (Kerr)

Capacity Limits Signal launch power [dBm]  Fiber nonlinearity Noise Ref: IEEE, vol.100, No.5 May 2012 Signal launch power [dBm] 

… Moore’s Law for Ever… ? Courtesy of Per O. Andersson

Multiplexing Techniques

100G Fiber Optic Transmission :: DP-QPSK DP-QPSK: Dual Polarization Quadrature Phase Shift Keying DP-QPSK is a digital modulation technique which uses two orthogonal polarization of a laser beam, with QPSK digital modulation on each polarization QPSK can transmit 2 bits of data per symbol rate, DP-QPSK doubles that capacity For 100Gbps, DP-QPSK needs 25G to 28G symbols per second. Electronics have to work at 25 to 28 GHz

BPSK- Binary Phase Shift Keying BPSK transmits 1 bit of data per symbol rate, either 1 or 0

QPSK- Quadrature Phase Shift Keying Use quadrature concept, i.e., both sine and cosine waves to represent digital data Two BPSK used in parallel Cosine wave

DP-QPSK in Fiber Optic Transmission DP-QPSK transmits 4-bits of data per symbol rate Sine wave Data stream Vertical polarized Cosine wave Laser source is linearly polarized Assume horizontal polarized laser source Horizontal polarized

Outline Challenges Multiplexing Techniques Routes to Longer Reach Distributed Amplification Hollow Core Fibers Routes to Higher Transmission Capacity Space Division Multiplexing (SDM)

Routes to Longer Reach Deal with low SNR Advance FEC More power efficient modulations format Maintain a high SNR Ultralow noise amplifiers Distributed amplification Deal with more nonlinearities Digital back-propagation Reduce the nonlinearity Install new large-area or hollow-core fibers

Distributed Amplification High SNR but will excite nonlinearities SNR degrades due to shot noise no issues of nonlinearity Raman pump power= 700 mW EDFA gain=20 dB, NF=3 dB Courtesy: Peter Andrekson, Chalmers Uni. Ideal distributed amplification (constant average signal power in the entire span) PSA: Phase sensitive amplifier with noise free gain medium

New Telecom Window at 2000 nm Hollow-Core Fibers Guiding by Photonic Bandgap Effect Key potential attributes: Ultra-low loss predicted near 2000nm (not single mode operation) (~ 0.05 dB/km predicted opt. Express, Vol.13, page 236, 2005) Very wide operating wavelength range (700 nm) Very small non-linearity: 0.001 x standard SMF Lowest possible latency Distributed Raman amplification may be challenging, however.

Hollow-Core Fiber :: SNR Comparison of ultralow loss (0.05 dB/km) hollow-core fiber and EDFA In conventional fiber (0.2 dB/km) Courtesy: Peter Andrekson, Chalmers Uni.

Hollow-Core Fiber :: SNR Comparison of ultralow loss (0.05 dB/km) hollow-core fiber, EDFA and distributed Raman amplification in conventional fiber (0.2 dB/km) Span loss: 20 dB Backward Raman (100 km) Bidirectional Raman (100 km) (10 + 10 dB) Courtesy: Peter Andrekson, Chalmers Uni. A low-loss hollow core fiber with EDFA spacing of 400 km performs similar to backward pumped Raman system with 100 km pump spacing

Spectral Efficiency Impact of Nonlinear Coefficient + 2.2 b/s/HZ for each X 10 Gamma reduction Ref: R-J. Essiambre proc. IEEE vol. 100, p. 1035, 2012

Thulium-Doped Silica Fiber Amplifiers (TDFA) at 1800-2050 nm ECOC 2013 Paper Tu.1.A.2 Suitable with low-loss hollow core transmission fiber Very wide operation range (> 200nm) Noise figure ~ 5 dB Laser diode pumping at 1550 nm 100 mW saturated output signal power

Outline Challenges Multiplexing Techniques Routes to Longer Reach Distributed Amplification Hollow Core Fibers Routes to Higher Transmission Capacity Space Division Multiplexing (SDM)

Routes to Higher Transmission Capacity CLB= N * B * log2(1+SNR) Overall transmission capacity: Available optical bandwidth (B) New amplifiers Extend low-loss window X Spectral efficiency (bit/sec/Hertz) Electronics signal processing Low nonlinearity Number of channels (N) Install new multi-core/multi- mode fibers

Typical Attenuation Spectrum for Silica Fiber Only 8-10 % is utilized in C band With SE of 10 per polarization a fiber can support well over a Pb/s

Space Division Multiplexing (SDM)

Inter-Core Crosstalk (XT)

Inter-Core Crosstalk (XT)

From WDM Systems to SDM & WDM Systems Flexible upgrade: Add transponder in lambda and M

State of the Art Systems